Transcript RGM Coastal processes

Location map of Sydney’s Northern beaches
COASTS – Issues & Processes
Who & what?
Biophysical interactions:
how do the four spheres impact
upon coasts?
Atmospheric interactions with the coast:
• Winds generate waves, storm surges and
currents.
• Temperature affects the rate of weathering and
the amount of river sediment.
• Glaciation (ice age) brings changes in sea
levels.
• Precipitation affects channel flow, groundwater
and supply of terrestrial (land) sediment.
• Human interactions have caused Global
warming and this is predicted to cause sea level
rising.
Lithospheric interactions with the coast:
• Tectonic forces cause crustal uplift and
subsidence, volcanic activity, earthquakes and
tsunamis.
• Different rock types and structures have varied
rates of weathering and erosion, ie soft and hard
rock. This also effects rates of mass movement.
• Soil degradation, erosion, salinisation,
• Humans have built structures such as sea walls
and breakwaters to stabilise shoreline
conditions. Humans dredge sand bars (to get rid
of excess sand that block lagoons)
Hydrospheric interactions with the coast:
• Together with the atmosphere the
hydrosphere creates currents, tides, storm
surges, sedimentation, groundwater,
channel flow, surface runoff, rivers, lakes,
lagoons.
• Human interactions effect flooding, river
flow, urban runoff and the storage of
water..
Biospheric interactions with the coast:
• The variance in dune vegetation affects
the stability of dunes.
• Healthy dune systems should sustain a
diversity of Flora and Fauna.
• Human interaction can effect this health,
through fires, development, deforestation
and introduced species.
Coasts - Energy at the Coast
The coast represents the meeting point
between the land and sea. Coasts are
very dynamic areas and they are
constantly changing. This change is due
to 3 main processes which operate at the
coast,
1. Erosion;
2. Transport and
3. Deposition.
Examples of coastal landforms formed
by erosion and deposition
Agents of Erosion
Waves
The Zones of the Coast
• These 3 processes are all driven by the
amount of energy that is available at the
coast. The main agents of change at the
coasts are waves. Waves are movements
of energy throughout the water, but where
do waves get their energy from? The
answer to this is wind.
Waves
• There are 3 main factors which will affect
the strength of a wave and therefore
whether it is more likely to erode or build
up the coastline
Waves
• (i) the strength and speed of the wind the faster the wind, the more energy is
transferred and therefore the bigger the
wave that is produced.
Waves
• (ii) the duration of the wind - this is the
length of time for which the wind has
blown - the longer the wind blows, the
more energy is transferred to the wave
Waves
• (iii) the fetch - this is the distance over
which the wind has blown and therefore
how far the wave has travelled. The longer
the fetch, the larger the wave is likely to
be.
• How is energy transferred to create
waves?
1. Wind creates friction on the waters
surface;
2. Frictional drag between the wind and the
waters surface causes water particles to
rotate and energy is transferred forward;
3. When the wave reaches shallow water, it
slows down due to friction between the
base of the wave and the sea bed. The
shape of the wave becomes increasingly
elliptical;
4. The top of the wave continues to move
forward as it is unaffected by the friction
with the sea bed. It becomes steeper and
steeper and eventually breaks;
5. Water moves up the beach as the swash;
6. Water then returns back down the beach
as the backwash.
• As wind blows over the surface of the sea,
it creates friction. This frictional drag
causes water particles to begin to rotate
and energy is transferred forward in the
form of a wave. Whilst the water moves
forward, the water particles return to their
original position.
• As a wave reaches shallow water, friction
between the sea bed and the base of the
wave causes the wave to begin to slow
down and its shape becomes more
eliptical. The top of the wave however,
unaffected by the friction, becomes
steeper until it eventually breaks.
When waves reach the shore
• When the wave breaks, water washes up
the beach, this is called the swash. The
movement of water backdown the beach is
called the backwash.
Rates of erosion
• It is the rate at which waves reach the
coast which determine whether the main
process acting on the coastline is erosion
or deposition. There are two main types of
waves:
Constructive waves
CONSTRUCTIVE WAVES
• Tend to arrive at the coast at a rate of less
than 8 waves per minute, they are low
energy waves and are small in height.
They have a strong swash and a weak
backwash. This means that constructive
waves tend to deposit material and build
up a beach.
•
•
•
•
•
•
•
•
small in height
gentle angle
less energy
strong swash and weak backwash (material is
moved up the beach by the strong swash)
ANIMATION
http://www.curriculumbits.com/prodimages/detail
s/geography/geo0003.html
VIDEO
http://www.youtube.com/watch?v=EpFykGLsDn
M
DESTRUCTIVE WAVES
DESTRUCTIVE WAVES
• Have much higher energy and tend to
arrive at the coast at a rate of more than 8
per minute. They are much larger in height
often having been caused by strong winds
and a large fetch. These high energy
waves have a weak swash but a strong
backwash, which erode the beach but
pulling sand and shingle down the beach
as water returns to the sea.
•
•
•
•
•
•
•
•
large in height
steep
lots of energy
weak swash and strong backwash (beach is
scoured and degraded as the strong backwash
pulls sand and shingle back down the beach)
ANIMATION
http://www.curriculumbits.com/prodimages/detail
s/geography/geo0003.html
VIDEO
http://www.youtube.com/watch?v=FHvAxYrHdD
U&eurl=http://burychurchgeog.blogspot.com/200
7/09/waves.html
Wave animations
• http://www.schoolportal.co.uk/GroupDownloadFile.asp?file=
21401 Summer/winter pattern
Fill in the table
Constructive
Wave height
Wave steepness
Wave length
Wave period
Frequency
High or low energy
Beach gradient
Stronger swash or
backwash
Destructive
Waves
• Wave Machine Simulator - create your
own ocean wave
• Waves - includes animation of swash
and backwash (BBC Bitesize)
Key Term Check:
• Swash - the movement of water and material up
the beach (in direction of prevailing wind)
Backwash - the movement of water and material
back down the beach (straight back down due to
gravity
Constructive wave - low energy wave with
greater swash than backwash - tends to build up
the beach
Destructive wave - high energy wave with
greater backwash than swash - tends to erode
beach
Wave of translation animation
• http://www.saddleback.edu/faculty/csolem/
Lecture%2012/anim04.swf
Wave terminology
(www.class.unl.edu)
Fetch
Plunging breakers
Plunging breaker
Spilling breakers
Spilling breaker
Wave refraction animation
• http://www.saddleback.edu/faculty/csolem/
animations/38_WaveMotion.swf
Wave animations
• Home 1.Beach anatomy, 2.wave anatomy,
3.wave motion, 4.wave refraction,
5.longshore current, 6.rip current,
7.coastal deposition, 8.coastal erosion
landforms, 9.coastal erosion: controls
Models
• http://www.uky.edu/AS/Geology/howell/go
odies/elearning/module14swf.swf
Tides defined
• "Tides" is a generic term used to define
the alternating rise and fall in sea level
with respect to the land, produced by the
gravitational attraction of the moon
and the Sun.
(http://www.bigelow.org)
Spring & neap tides
• Happen on a 28 day
•
•
cycle, not annually!
Spring tides lead to a
large tidal range, the
highest tides and the
lowest tides.
Neap tides have a
small tidal range.
(www.alunatime.org)
Tides and erosion
• Neap tides lead to concentrated erosion on one part of
•
the cliff.
Tides are not directly linked to wave size (wind) so you
can get some of the most powerful waves combining
with neap tides.
(www.the(www.the-riverriver-thames.co.uk)
Tides animation
• http://www.ioncmaste.ca/homepage/resour
ces/web_resources/CSA_Astro/files/conte
nt/multimedia/unit3/tides/tides.swf
• http://www.onr.navy.mil/focus/ocean/motio
n/tides1.htm
Coastal Processes: Erosion,
Transport and Deposition
• There are 3 main processes that cause a
coastline to change:
1. Erosion
2. Transport
3. Deposition.
• There are number of factors which affect
each of these processes - we are going to
start by exploring erosion processes and
the factors that can affect the amount of
erosion that may take place along a
coastline.
COASTAL EROSION
• Erosion Processes:
Erosion is the wearing away of rocks, at
the coast there are 6 main types of erosion
processes in action (see animations on the
following website)
• http://www.bbc.co.uk/schools/gcsebitesize/
geography/coastal/coastalprocessesrev4.s
html
ABRASION
1. ABRASION (this is also known as
corrasion) - this is where rock fragments
are hurled at cliffs by breaking waves,
gradually scraping away at the cliff face;
HYDRAULIC ACTION
2. HYDRAULIC ACTION - as waves break
against the cliff face, the pressure of the
breaking wave can compress air in cracks.
This compressed air gradually forces open
the crack in the rock - as this process
continues, the rock becomes increasingly
weakened.
Wave approaches the cliff. Note cracks exaggerated In Wave reaches the cliff & the air trapped by
size
the wave is compressed into the crack.
Wave rebounds from the cliff & the compressed air
escapes explosively, enlarging the cracks & ripping
bits of rock off.
HYDRAULIC ACTION
CORROSION
3. CORROSION (this is also known as
solution) - this occurs where the salt
water is able to dissolve some of the
chemicals in rocks
SCOURING
• 4. SCOURING - this occurs at the base of
the cliff as the waves break and swirl
around, gradually removing loose rock.
ATTRITION
5. This is where rock fragments carried by
the waves hit against each other and
gradually wear down to form sand and silt
WAVE POUNDING
6. The sheer force of waves hitting against
the cliff face
Video of headland erosion over a year
http://www.youtube.com/watch?v=ChEHQU
MEkXw
Coastal Kung Fu
• http://blip.tv/scripts/flash/blipplayer.swf?aut
oStart=false&file=http://blip.tv/file/get/Adta
stic2001-CoastalKungFu837.flv?source=3
These processes of erosion
form a series of distinctive
landforms at the coast
Rates of Coastal Erosion
• So what are the factors that determine
how much erosion can take place at the
coast?
The Resistance of the Rocks
• - e.g. sandstone, limestone, chalk and
granite are resistant rocks (often forming
cliffs and headlands) and erode relatively
slowly, whilst less resistant rocks such as
clay and shale are easily eroded.
The Strength of the waves
• affected by the wind strength and duration
and its fetch
The shape of the coastline
• (which is dependent on its geology) - on
concordant coastlines, rocks are parallel to
the wave front and therefore rates of
erosion are similar along the coastline. On
discordant coastlines, differential erosion
may occur, where bands of hard and soft
rock outcrop at right angles to the sea.
Consequently headlands and bays form
along discordant coastlines and whilst
headlands remain exposed to the force of
the waves, bays are sheltered.
COASTAL TRANSPORT
• The second process operating at the coast
is transport. Material eroded by the sea is
carried within the water in a number of
ways, minerals dissolved from rocks are
carried in solution, whilst small rock
fragments, light enough to be held within
the water, float in suspension.
COASTAL TRANSPORT
• The largest rock fragments which are too
heavy to be picked up by the waves, are
transported by the process of traction,
this is where they roll along the bed when
the waves pick up enough energy.
COASTAL TRANSPORT
• Finally, medium sized rock particles, which
cannot be carried by the waves all the
time, are moved by saltation. This is
where during times of higher wave energy
the particles are picked up and then
dropped again as the wave looses its
energy.
LONGSHORE DRIFT
• The main form of transport operating at
the coast is that of LONGSHORE DRIFT.
Longshore drift is the process by which
sand and pebbles are moved along a
beach by the movement of the waves.
• Amounts of Longshore Drift depend
on:
• The strength of the waves, size and
amount of material available for the
movement.
• Incline (slope) of the beach
COASTAL DEPOSITION
• Material is moved up the beach by the
swash at an angle which is controlled by
the prevailing wind. The backwash then
carries material back down the beach at
right angles to the coastline under the
influence of gravity. Gradually the material
is moved along the coastline, its direction
being controlled by the prevailing wind
direction.
COASTAL DEPOSITION
• Although some material eroded at the
coast is washed out to sea, most of the
material is transported along the coast by
longshore drift. Deposition will occur when
the waves are no longer able to transport
material due to a loss of energy.
COASTAL DEPOSITION
• This is the case with constructive waves,
where material is moved up the beach in
the strong swash, but the weak backwash
means material is deposited to build up
the beach
COASTAL DEPOSITION
• Deposition commonly occurs:
• where the water is sheltered (e.g. a bay)
and the waves lack energy where the
coast is shallow and the increased friction
between the water and the sea bed
reduces the energy available for
transport.
COASTAL DEPOSITION
• Deposition of material results in the
formation of a number of distinctive
features: beaches; spits; bars and
tombolos.
COASTAL DEPOSITION
• The final process operating at the coast is that of
deposition - this is where material that is too
heavy to be transported any more is left behind,
building up the beach. Due to the importance of
energy in transporting sand and shingle, it is the
largest material that is deposited first. A number
of distinctive features may form due to coastal
deposition.
• http://www.bbc.co.uk/schools/gcsebitesize/geogr
aphy/coastal/coastalprocessesrev5.shtml
Key Terms Check:
• Erosion - the wearing away and removal of material
Deposition - the dropping of material
Abrasion - the wearing of rock due to rock fragments
being hurled against cliffs
Attrition - the breakdown of rocks as they hit against
each other
Hydraulic Action - the force of waves causing rocks to
split apart as waves compress air in cracks in the rocks
Wave Pounding - sheer force of water hitting rocks
Solution - where minerals in rocks are dissolved by the
action of sea water
Scouring - occurs where water and broken rock
fragments swirl around at the base of cliffs gradually
wearing rock away.
Longshore Drift - the movement of material along a
coastline
Coastal Erosion Landforms Features and Formation
• Coastal Erosion Features
There are 3 main groups of coastal
features which result from coastal erosion:
1. Headlands and Bays
2. Caves, Arches, Stacks and Stumps
3. Cliffs and Wave-cut platforms
1. HEADLANDS AND BAYS
• Headlands are resistant outcrops of rock
sticking out into the sea, whilst bays are
indents in the coastline between two
headlands.
So how do headlands form?
• - Headlands form along discordant coastlines in
which bands of soft and hard rock outcrop at
right angles to the coastline.
- Due to the presence of soft and hard rock,
differential erosion occurs, with the soft, less
resistant rock (e.g. shale), eroding quicker than
the hard, resistant rock (e.g. chalk)
So how do headlands form?
• - Where the erosion of the soft rock is
rapid, bays are formed
- Where there is more resistant rock,
erosion is slower and the hard rock is left
sticking out into the sea as a headland.
- The exposed headland now becomes
vulnerable to the force of destructive
waves but shelters the adjacent bays from
further erosion.
2. CAVES, ARCHES, STACKS
and STUMPS
• Once a headland has formed it is then
exposed to the full force of destructive
waves and it gradually begins to erode.
you need to be able to describe the
erosion of a headland and the features
that form.
So how does a headland erode and caves,
arches, stacks and stumps form?
• Firstly, the sea attacks the foot of the cliff
and begins to erode areas of weakness
such as joints and cracks, through
processes of erosion such as hydraulic
action, wave pounding, abrasion and
solution;
• - Gradually these cracks get larger,
developing into small caves;
- Further erosion widens the cave and
where the fault lines runs through the
headland, two caves will eventually erode
into the back of each other forming an
arch, passing right through the headland.
• A combination of wave attack at the base
of the arch, and weathering of the roof of
the arch (by frost, wind and rain), weakens
the structure until eventually the roof of the
arch collapses inwards leaving a stack, a
stack is a column of rock which stands
separate from the rest of the headland.
- The stack will continue to erode,
eventually collapsing to form a stump
which will be covered by water at high tide.
Natural Arch
• Eventually the cave
deepens enough for
it to pass through
the headland, or it
meets another cave
coming the opposite
direction.
• London Bridge,
Australia
Stack
• Continual erosion of
the arch causes the
roof to become
unstable & collapse
• The Twelve
Apostles, Australia.
• How did the original headland shape
become eroded to the present coastal
landscape?
• A number of stages are involved.
• All rocks have lines of weakness. The sea
and its waves use hydraulic action,
abrasion, attrition and solution to erode
along any lines of weakness. Undercutting
takes place all around the headland.
• These lines of weakness get enlarged and
develop into small sea caves.
• The caves are deepened and widened on both sides of
the headland until eventually the sea cuts through the
headland, forming an arch.
• The rock at the top of the arch becomes unsupported as
the arch is enlarged, eventually collapsing to form a
stack.
• The stack gets eroded until only a stump remains.
• Over time the stump will disappear.
• As the headland retreats under this erosion, the gently
sloping land at the foot of the retreating cliff is called a
wave-cut platform.
Stages in coastal
Development
• a) geological weakness (e.g. fault) forming a
geo.
• b) formation of sea cave by marine erosion.
c) enlargement of cave to form arch.
• d) collapse of arch to form stack.
e) removal of stack to create a stump.
3. CLIFFS AND WAVE-CUT
PLATFORMS
• Cliffs are steep rock faces along the
coastline, they tend form along concordant
coastlines with resistant rocks parallel to
the coast.
So how do cliffs and wave-cut
platforms form?
• The erosion of a cliff is greatest at its base
where large waves break - here hydraulic
action, scouring and wave pounding
actively undercut the foot of the cliff
forming an indent called a wave-cut notch
whilst the cliff face is also affected by
abrasion as rock fragments are hurled
against the cliff by the breaking waves.
• This undercutting continues and eventually
the overhanging cliff collapses downwards
- this process continues and the cliff
gradually retreats and becomes steeper.
- As the cliff retreats, a gently-sloping
rocky platform is left at the base, this is
known as a wave-cut platform which is
exposed at low tide.
Smartboard Interactive
• Coastal_erosion_feat
ures_summary.noteb
ook
Coastal Deposition Landforms:
Features and Formation
• Material that is transported by the waves
along a coastline is eventually deposited
forming distinctive deposition features.
There are four main deposition features
that you need to learn the formation of.
These are:
Depositional Landforms
• These are:
1. Beaches
2. Spits
3. Bars
4. Tombolos
Beaches
• A beach is defined as the gently sloping area
of land between the high and low water
marks. Remember, beaches are not
permanent features as their shapes are
altered by waves.
• Beaches are the main feature of deposition
found at the coast, these consist of all the
material (sand, shingle etc.) that has built up
between the high and low tide mark.
• Beaches are made up of material lying between
the high and low tide mark. There are four main
sources of beach material.
• 1. Material deposited at the mouth of rivers
• 2. Cliff erosion, provides rock fragments that will
build up the beach
• 3. Constructive waves have a strong swash
pushing sands / pebbles up the beach
• 4. Longshore drift carries material from
elsewhere along the coast
• As constructive waves build up beaches,
they often form ridges in the beach known
as berms. The berm highest up the beach
represents the extent to which the water
has reached during high tide.
SPITS
• Spits are long narrow ridges of sand and
shingle which project from the coastline
into the sea.
• How are Spits formed?
SPITS
• Longshore drift transports material along
the coastline. Spits are formed in areas of
relatively shallow and sheltered water
where there is a change in the direction of
the coastline. Deposition occurs resulting
in the accumulation of sand and shingle.
SPITS
• The material initially deposited is the
largest material, dropped due to the
reduction in energy. Finer material is then
deposited, helping to build up the rest of
the spit. As the spit continues to grow
outwards, a short term change in wind
direction may result in a change in the
direction of the spit forming a curved end
(recurved laterals).
SPITS
• If growing across a river estuary, the
length of the spit will be restricted by the
river outlet washing sediment away. A salt
marsh may form in the sheltered, low
energy zone behind the spit.
BARS
• These form in the same way as a spit
initially but bars are created where a spit
grows across a bay, joining two
headlands. Behind the bar, a lagoon is
created, where water has been trapped
and the lagoon may gradually be infilled as
a salt marsh develops due to it being a low
energy zone, which encourages
deposition.
BARS
• Over time the lagoon will become infilled
by deposition as either rivers flows in to it
depositing material, or as waves break
over the top of the Bar depositing coastal
material
TOMBOLOS
• What are tombolo's and how are they
created?
• Tombolo's are ridges of sand and shingle which
join the mainland to an island.
• Tombolo's are created through the process of
longshore drift. Where there is a change in the
shape of the land, a spit forms in the shallow /
sheltered water. A tombolo is formed where the
spit continues to grow until it reaches an island,
forming a link with the mainland.
Coastal Defence
• When managing the coastline there are
two main options:
1. HARD ENGINEERING
This is where man made coastal defence
structures are used to reflect large
amounts of wave energy and hence
protect the coastline.
SOFT ENGINEERING
2.This is where beaches or naturally formed
materials are used to control / re-direct
erosion processes. Eg a sand dune.
1. Re-Curved Sea Wall
Re-Curved Sea Wall
• - concrete wall which is curved on the underside
to deflect the power of the waves
• these can be very expensive and the deflected
waves can scour material at the base of the wall
causing them to become undermined
these are however a very effective means of
preventing erosion and they reflect rather than
absorb wave energy.
2. Rip Rap
Rip Rap
• large boulders on the beach absorb wave
energy and break the power of the waves
- although movement of the boulders is
expensive this can be a much cheaper method
than some other solutions
- the boulders can however be undermined
easily by waves washing away sand and shingle
beneath them. They also can be quite ugly,
changing the appearance of a coastline.
Groynes
3. Groynes
• these structures (usually either wooden or
steel) are designed to top longshore drift
and therefore act to build up and anchor
beach material, protecting the base of
cliffs.
Groynes
• they are effective at reducing erosion in the area
they are constructed in by causing significant
build up of beach material
- groynes may however starve areas further
down the coast of material by stopping
longshore drift, resulting in an increase in
erosion in these areas
4. Gabions
Gabions
• these cages of boulders are built into cliff
faces to protect the cliff from the force of
the waves;
- they are cheaper than sea walls and can
be very effective where severe erosion is a
problem
- they are however visually intrusive
5. Revetments
Revetments
• these wooden structures break the force of
waves and beach material builds up behind
them
- they are cheap and effective at breaking waves
- as well as being visually intrusive however they
do need replacing more frequently than most
other defence methods.
Soft Engineering Techniques
Soft Engineering Techniques
• Soft engineering includes beach replenishment
in which beach material is added to provide a
"natural solution". Environmentally this is a
preferred option as it maintains the beauty of the
landscape and avoids visual intrusion, however
it can be expensive to maintain as longshore
drift continues to move beach material down the
coast and therefore regular replenishment is
required.
• Sand Dunes and salt marshes can also be
encouraged to act as natural barriers to the
waves.
COASTAL
MANAGEMENT
Groynes
Gabion Cages
Gabion Cages
Rip Rap
Sea Wall
Sea Wall
Sea Wall
Revetment
Beach Feeding